Conventionally, at the time of engine ignition, a method of igniting a mixture gas using a spark plug (spark injection (SI) plug) is employed (see patent document 1, for example). The SI plug is a well-known electric component used to ignite the mixture gas in a combustion chamber of an engine, and generally includes a center electrode, an insulator provided around an outer circumference of the center electrode, a mounting screw part provided around an outer circumference of the insulator and mounted on the engine, and a ground electrode connected to an end part of the mounting screw.
When a high voltage is applied between the center electrode and the ground electrode of the SI plug, insulation between the electrodes is destroyed and a discharge phenomenon is generated, and an electric spark is generated. The mixture gas can be ignited by energy of this electric spark.
[Patent Document 1] Japanese Patent Application Laid-Open No. 64-36981